Sound for motion picture films has traditionally been recorded on the film in an analog fashion. That is, a sound track, having a variable width which corresponds to the amplitude of the sound, runs the length of the film between the image frames and the sprocket holes. Compared to current audio reproduction standards, the quality of motion picture sound reproduced in this manner is relatively poor (its quality being slightly better than that produced by AM radio).
With the advent of digital sound reproduction using compact discs and digital audio tape, it has been desirable to obtain the equivalent high quality sound reproduction in movie theaters. One persistent roadblock to achieving this goal has been the inability to efficiently pack enough digital data into the space available on a motion picture film to faithfully reproduce multiple audio channels.
In the making of a motion picture film having a digital sound track, once the audio signals are digitized, they are encoded and formatted and are optically applied to the motion picture film During playback of the film in a movie theater, the digital data in the sound track is optically sensed and is decoded to reproduce the original audio signals accurately. In order to faithfully reproduce the original audio input attention must be given to the quantization level and sampling rate of the digitizer and the number of redundant data bits that are added to the sound track for error detection and correction.
In modern movie theaters, it has become a standard practice to implement more than just a two channel audio system. For example, in addition to the standard left and right channels, a center channel is output through a speaker behind the movie screen, two or more surround channels are output at various areas in the theater to create the illusion of motion, and a specialized "subwoofer" channel is used to recreate very low audible and inaudible frequencies. Thus, a minimum of six audio channels, five of them having a full audible bandwidth, must be digitally recreated. The problem is intensified by the limited space available for the data on motion picture film and the physical limitations of the film resolution which affects the minimum size of each digital bit represented on the film. Further, in order to faithfully reproduce sound in a digital system, error correction must be utilized, thus adding more data to the already limited sound track area